Jet Trails

The appearance of trails behind jets is caused by one of two things:

"Low Pressure Zone": Water vapor from the air that condenses in the low-pressure zone of wing tips or flaps; or,
"Ice Crystals": Ice at high altitudes forming around moisture from water injected into jet exhaust by old, rarely-used, obsolete engines..

"Low Pressure Zone" Trails

"Low Pressure Zone" trails only occur when humidity is very high. They only last as long as a negative pressure zone persists. Below is a photo of a typical LPZ trail.

In this picture, you will notice that the trail dissipates almost completely just a few feet behind the wing. This occurs because the moisture in the trail is reabsorbed back into the air as quickly as the air pressure increases towards normal. The trail in the picture is not visible from the ground.

I've seen this principle demonstrated when a vacuum is applied in a glass jar in a high-school science class. Sometimes, you will see condensation form in the jar as the vacuum increases, but when the vacuum is released, all of the condensation dissipates immediately.

The picture above was used on Wikipedia as the definitive example of a "contrail", It was replaced in the late 2000's by pictures of trails coming out of jet engines.

It's very interesting that the writers did not use pictures of trails coming from jet engines, but it's understandable because these trails simply did not exist, except for the very rare example outlined in the next section.

"Ice Crystals" from Water Injection Engines

A few older jets, namely the BAC One-Eleven, uses the Spey 512 water injection engine. This engine injects water into the core to increase thrust. When older jets using this now-rare engine fly at high altitudes in very cold air, the emitted water vapor may condense and freeze into small ice crystals. These tiny ice crystals become visible, but when they are exposed to the sun, they quickly melt dissipating within seconds.

These trails are so thin and short-lived that they have rarely been seen at any altitude. Furthermore, because this phenomenon is extremely rare and because it only occurs at higher altitudes, these types of trails are simply not visible from the ground.

Today, there are only about 50 BAC One-Elevens in service and virtually all of them in Europe.

Historical Rarity of Contrails

Both of the trails described above are called "contrails".

Contrails are extremely rare, but on the rare occasion that one does form at high altitudes, it dissipates quickly and cannot be seen from the ground. It's simply too high and too small and thin.

I have been a sky-watcher nearly all of my life. I grew up near some of the most heavily-traveled airspace in the world. I also lived on a military base. I have traveled across the United States visiting over 20 states multiple times.

Throughout the past 30 years, all across this country, I have watched thousands upon thousands of jets fly across the sky. Most of them were commercial jets, others were military, but they all had one thing in common:

They never left a trail.

Yes, after 30 years of sky-watching, I have never seen a single jet leave a trail... until now. Now I see 5-times more jets in the sky, and 49 out of 50 of them have trails.

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